The grain oriented silicon steel sheets can be utilized as cores for transformers and other electrical machinery and equipment, and are required to have a high magnetic flux density (represented by their B.sub.10 value) and a low iron loss (represented by their W.sub.178/50 value).
Up to the present, there have been many attempts for achieving the above requirement, and grain oriented silicon steel sheets having a low iron loss with a magnetic flux density, a B.sub.10 value of not less than 1.89T and an iron loss, and a W.sub.17/50 value of not more than 1.05 W/kg are manufactured today.
However, the production of a grain oriented silicon steel sheet having a lower iron loss has become an urgent problem bordering on the energy crisis. In this connection, a system of granting a bonus on super-low iron loss silicon steel sheets (Loss evaluation system) is widely spread in Europe and America.
Recently, the following methods are proposed for producing grain oriented silicon steel sheets having a considerably reduced iron loss value.
That is, as disclosed in each of Japanese Patent Application Publication No. 57-2,252, Japanese. Patent Application Publication No. 58-53,419, Japanese Patent Application Publication No. 58-5,968, Japanese Patent Application Publication No. 58-26,405, Japanese Patent Application Publication No. 58-26,406, Japanese Patent Application Publication No. 58-26,407, and Japanese Patent Application Publication No. 58 36,051, an artificial grain boundary is introduced into the surface of the grain oriented silicon steel sheet by utilizing an AlN precipitation phase as an inhibitor for inhibiting the growth of crystal grains in an unsuitable direction at finish annealing and irradiating a laser beam onto the steel sheet surface at an interval of several mm in a direction substantially perpendicular to the rolling direction to thereby reduce the iron loss through the artificial grain boundary.
In such a method of introducing an artificial grain boundary, however, regions of high transformation density are locally formed, so that there is a problem that the resulting products are stably used only at a low temperature below about 350.degree. C.
In the production of the grain oriented silicon steel sheet utilizing the AlN precipitation phase as mentioned above, it is necessary to conduct the heating of the slab before hot rolling at a temperature higher than that of ordinary steel for the dissociation and solution of MnS coexistent with AlN as an inhibitor, but when the slab heating is carried out at such a high temperature, hot tearing is caused at the slab heating or hot rolling stage to cause the occurrence of surface defects in the product, and particularly the surface properties of the product are considerably degraded when the content of Si obstructing the hot workability exceeds 3.0%.
In this point, as disclosed in Japanese Patent Laid open No. 59-85,820, the inventors have noticed that when utilizing the AlN precipitation phase, a silicon steel material having a high Si content of Si: 3.1.about.4.5% is essentially a material suitable for obtaining a high magnetic flux density, low iron loss product, and have found that the surface properties can be made good even at the high Si content by enriching the Mo content in the surface layer of the steel material before the hot rolling as a means for solving the problem of degradation of surface properties. According to this means, the surface properties of the product are largely improved as compared with the former case, but if it is particularly intended to thin the gauge of the product to 0.23.about.0.17 mm for obtaining low iron loss, there remains a large problem that the improvement effect on the surface properties is small.
Aside from this, the utilization of an AlN precipitation phase is naturally dependent on a strong one-stage cold rolling process, so that if it is intended to manufacture a thinned product, the secondary recrystallized grains become very unstable, and it is difficult to grow the secondary recrystallized grains highly aligned in Goss orientation.
Lately, Japanese Patent laid open No. 59-126,722 discloses that in order to stably manufacture thinned products by utilizing an AlN precipitation phase at high Si content, a two-stage cold rolling process largely different from the conventional strong one-stage cold rolling process may particularly be applied to a hot rolled material containing small amounts of Cu and Sn in addition to AlN.
This is effective for stably reducing the iron loss of the thinned product, but has yet many problems is that it is difficult to obtain products having excellent surface properties because high-temperature heating of the slab is usually required with increased Si and that the cost of the product becomes considerably higher because small amounts of Sn and Cu are added for stabilizing secondary recrystallized grains.
As a method of reducing the iron loss of the grain oriented silicon steel sheet, there are fundamentally considered the following methods;
1 the increasing of Si content in silicon steel; PA1 2 the thinning of product gauge; PA1 3 increasing the purity of the steel sheet; PA1 4 the growing of secondary recrystallized fine grains without lowering the degree of alignment of the secondary recrystallized grain in Goss orientation in the product. PA1 Si: 3.1.about.4.5 wt %, PA1 Mo: 0.003.about.0.1 wt %, PA1 acid soluble Al: 0.005.about.0.06 wt %, and PA1 at least one of S and Se: 0.005.about.0.1 wt % in total to hot rolling to form a hot rolled steel sheet; PA1 Si: 3.1.about.4.5 wt %, PA1 Mo: 0.003.about.0.1 wt %, PA1 Sb: 0.005.about.0.2 wt %, PA1 acid soluble Al: 0.005.about.0.06 wt %, and PA1 at least one of S and Se: 0.005.about.0.1 wt % in total to hot rolling to form a hot rolled steel sheet; PA1 Si: 3.1.about.4.5 wt %, PA1 Mo: 0.003.about.0.1 wt %, PA1 acid soluble Al: 0.005.about.0.06 wt %, and PA1 at least one of S and Se: 0.005.about.0.1 wt % in total to hot rolling to form a hot rolled steel sheet; PA1 Si: 3.1.about.4.5 wt %, PA1 Mo: 0.003.about.0.1 wt %, PA1 Sb: 0.005.about.0.2 wt %, PA1 acid soluble Al: 0.005.about.0.06 wt %, and PA1 at least one of S and Se: 0.005.about.0.1 wt % in total to hot rolling to form hot rolled steel sheet; subjecting the hot rolled steel sheet to primary cold rolling at a reduction of 10.about.60% and an intermediate annealing and a secondary cold rolling at a reduction of 75.about.90% to obtain a cold rolled thin sheet having a final gauge of 0.1.about.0.25 mm; subjecting the cold rolled thin sheet to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere, during which it is previously subjected to a treatment for the formation of heterogeneous microareas onto the surface of the thin sheet after the subsequent high-temperature finish annealing; and subjecting the thin sheet to high-temperature finish annealing. PA1 Si: 3.1.about.4.5 wt %, PA1 Mo: 0.003.about.0.1 wt %, PA1 acid soluble Al: 0.005.about.0.06 wt %, and PA1 at least one of S and Se: 0.005.about.0.1 wt % in total to a hot rolling to form a hot rolled steel sheet; subjecting the hot rolled steel sheet to primary cold rolling at a reduction of 10.about.60% and intermediate annealing and secondary cold rolling at a reduction of 75.about.90% to obtain a cold rolled thin sheet having a final gauge of 0.1.about.0.25 mm; subjecting the cold rolled thin sheet to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere; subjecting the thin sheet to high-temperature finish annealing; and forming heterogeneous microareas onto the surface of the thin sheet. PA1 Si: 3.1.about.4.5 wt %, PA1 Mo: 0.003.about.0.1 wt %, PA1 Sb: 0.005.about.0.2 wt %, PA1 acid soluble Al: 0.005.about.0.06 wt %, and PA1 at least one of S and Se: 0.005.about.0.1 wt % in total to hot rolling to form hot rolled steel sheet; subjecting the hot rolled steel sheet to primary cold rolling at a reduction of 10.about.60% and an intermediate annealing and secondary cold rolling at a reduction of 75.about.90% to obtain a cold rolled thin sheet having a final gauge of 0.1.about.0.25 mm; subjecting the cold rolled thin sheet to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere; subjecting the thin sheet to high-temperature finish annealing; and forming heterogeneous microareas onto the surface of the thin sheet.
At first, it has been attempted to increase the Si content to a value higher than the usual value of 3.0% as regards the method 1, or to thin the product gauge from the usual values of 0.35, 0.30 mm to 0.23, 0.20 mm as regards the method 2. In any case, however, problems are encountered in that the secondary recrystallized texture becomes non-uniform and the Goss orientation alignment lowers.
In addition, when the Si content is increased from the usual value according to the method 1, hot brittleness becomes conspicuous, and hot tearing is caused in slab heating or hot rolling to considerably degrade the surface properties of the product as previously mentioned.
On the other hand, the development of the improvement of steel sheet purity 3 or orientation 4 is considered to be extreme at the present. For example, the Goss orientation of secondary recrystallized grains in the existing products is aligned within 3.degree..about.4.degree. on average with respect to the rolling direction, so that it is very difficult in metallurgy to make the crystal grain small under such a highly aligned state.
Considering the recent trend of the aforementioned conventional techniques and the backgrounds of the above situations, it is an object of the invention to provide a method of stably and advantageously producing grain oriented silicon steel thin sheets having very excellent surface properties, a considerably small iron loss and a high magnetic flux density on an industrial scale.